Hammer device

ABSTRACT

A down-the-hole hammer includes a drill bit and a piston reciprocating therebehind to periodically strike the drill bit. The drill bit includes front and rear portions of difference impedance, and the piston includes front and rear portions of difference impedance. In the drill bit, the front portion has a larger impedance than the rear portion. In the piston, the rear portion has a larger impedance than the front portion.

BACKGROUND OF THE INVENTION

The present invention relates to a hammer device, preferably a down-the-hole hammer, including a casing, a piston, a drill bit and means for activating the piston to frequently strike the drill bit. The invention also relates to a piston and a drill bit per se.

In down-the-hole hammers the kinetic energy of the piston is transmitted by elastic waves through the drill bit and finally to the rock. However, said transmission is not carried out in an optimal way since the piston is not related to the drill bit in terms of length and mass. Also the drill bit does not cooperate with the rock in the best mode.

In prior art down-the-hole hammers very little attention has been paid to the adaption of the piston to the drill bit when said drill bit has a mass concentration at the end directed towards the rock.

The aim of the present invention is to further improve the energy transmission from the piston to the rock via the drill bit. This is realized by paying attention also to the distribution of the impedance in the piston and in the drill bit of a hammer device.

BRIEF DESCRIPTION OF THE DRAWINGS

Below an embodiment of a down-the-hole hammer according to the present invention is described, reference being made to the accompanying drawings, where

FIG. 1 schematically discloses the piston and the drill bit of a down-the-hole hammer according to the present invention;

FIG. 2 discloses the relationship between the applied force versus the penetration for a drill bit working a rock surface;

FIG. 3 discloses in a diagram the relationship between the degree of efficiency versus the relationship Z_(M) /Z_(T) ;

FIG. 4 discloses in a diagram the relationship between the degree of efficiency versus the relationship T_(M) /T_(T) ;

FIG. 5 discloses in a diagram the relationship between the degree of efficiency versus the parameter β; and

FIG. 6 discloses a diagram showing the compressive and tensile stresses in the piston and the drill bit.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

In FIG. 1 the piston 10 and the drill bit 11 are schematically shown. As is evident from FIG. 1 the piston 10 and the drill bit 11 have a reversed design relative to each other.

The piston 10 has two portions 10a and 10b. The portion 10a has the length L_(M1) and the impedance Z_(M1) while the portion 10b has the length L_(T1) and the impedance Z_(T1). The drill bit 11 has two portions 11a and 11b. The portion 11a, i.e. the head of the drill bit, has the length LM₂ and the impedance ZM₂ while the portion 11b, i.e. the shaft of the drill bit, has the length L_(T2) and the impedance Z_(T2).

When stress wave energy is transmitted through pistons and drill bits it has been found that the influence by variations in the cross-sectional area A, the Young's modulus E and the density ρ can be summarised in a parameter Z named impedance. The impedance Z=AE/c, where c=(E/ρ)^(1/2), i.e. the elastic wave speed. Any combinations of A, E and ρ that corresponds to a certain value of the impedance Z gives the same result in respect of stress wave energy transmission.

It should be pointed out that the impedance Z is determined in a certain cross-section transverse to the axial direction of the piston 10 and the drill bit 11, i.e. the impedance Z is a function along the axial direction of the piston 10 and the drill bit 11.

Therefore, within the scope of the present invention it is of course possible that the impedances Z for the different portions 10a, 10b, 11a and 11b may vary slightly, i.e. Z_(M1), Z_(T1), Z_(T2) and Z_(M2) do not need to have a constant value within each portion but can vary in the axial direction of said portions 10a, 10b, 11a and 11b. In the practical design of the piston 10 and the drill bit 11 the provision of e.g. circumferential grooves and/or splines are quite frequent. Also the provision of e.g. a circumferential shoulder may be necessary.

It should also be pointed out that even if e.g. the portions 10a and 10b must have different impedances Z_(M1) and Z_(T1), respectively it is possible to design the piston 10 with a generally constant cross-sectional area by using different materials in the portions 10a and 10b.

It is also necessary to define a further parameter, namely a time parameter T. The definition is T=L/c, where L is the length of the portion in question and c is the elastic wave speed in the portion in question. Thus for the portion 10a T_(M1) =L_(M1) /c_(M1), for the portion 11a T_(M2) =L_(M2) /c_(M2), for the portion 10b T_(T1) =L_(T1) /cT₁ and for the portions 11b T_(T2) =L_(T2) /c_(T2). The reason why it is necessary to have the time parameter T instead of the length L is that different portions may consist of different materials that have different values regarding the elastic wave speed c.

Within the scope of the present invention it is also possible that e.g. the portion 10a can consist of several sub-portions having different elastic wave speed c. In such a case the time parameter T is calculated for each sub-portion and the total value of the time parameter T for the entire portion 10a is the sum of the time parameters T for each sub-portion.

FIG. 2 shows the relationship between the force F applied to the rock versus the penetration u into the rock . The line k₁ illustrates the relation between the force F and the penetration u when a force F is loaded to the rock. Thus k₁ =F/u during the loading sequence and k₁ is a constant. The force F₁ corresponds to the penetration u₁. The unloading of the force F is illustrated by the line k₂. Thus k₂ =F/u during the unloading sequence and k₂ is a constant. When complete unloading has taken place there is a remaining penetration u₂ which means that a certain work has been carried out upon the rock, said work being illustrated by the triangular dotted area. The amount of work that said area represents is defined as W.

The kinetic energy of the piston 10 when moving towards the drill bit 11 is defined as Wk.

As stated above the aim of the present invention is to maximize the degree of efficiency, which is defined as the relationship W/W_(k).

The present invention is based on the idea that the mass distribution of the piston 10 is such that initially a smaller mass, i.e. the portion 10b is contacting the drill bit 11. Subsequently, a larger mass, i.e. the portion 10a, follows. It has turned out that by such an arrangement almost all of the kinetic energy of the piston is transmitted into the rock via the drill bit.

The most important parameter is the impedance ratios Z_(M1) /Z_(T1) and Z_(M2) /Z_(T2). Said parameter should be in a certain interval. In order to have an optimum degree of efficiency it is also important that the time parameter ratios T_(M1) /T_(T1) and T_(M2) /T_(T2) are in a certain interval.

In FIG. 3 a diagram shows the relationship between the degree of efficiency W/Wk versus the impedance ratio ZM/ZT, said ratio being valid for both the piston 10 and the drill bit 11. When setting up the diagram in FIG. 3 T_(M) /T_(T) =0.5 and β=1, see below concerning definition of β. As can be learnt from FIG. 3 the peak of W/W_(k) is within the interval 3.0-5.5, preferably 3.5-4.5 of Z_(M) /Z_(T). In said preferred interval the degree of efficiency W/W_(k) is higher than 95%. The highest degree of efficiency W/W_(k) is achieved when Z_(M) /Z_(T) =4.

Since the degree of efficiency W/W_(k) has its peak when Z_(M) /Z_(T) =4 it can be concluded that the theoretically preferred design is when the different portions 10a, 10b, 11a, 11b of the piston 10 and drill bit 11 each have a constant impedance Z in their axial directions. Also the portions 10a and 11a should have the same impedance and the portions 10b and 11b should have the same impedance. However, this is not likely to happen in the practical embodiments, see above. Therefore, it should again be emphasized that the impedances Z_(M1), Z_(T1), Z_(T2) and Z_(M2) need not have constant values but can vary in the axial direction of the corresponding portions 10a, 10b, 11a and 11b, respectively. The only restriction is that the ratios Z_(M1) /Z_(T1) and Z_(M2) /Z_(T2) are in the intervals specified in the appending claims.

In FIG. 4 a diagram shows the relationship between the degree of efficiency W/W_(k) versus the time ratio T_(M) /T_(T), said ratio being valid for both the piston 10 and the drill bit 11. When setting up the diagram in FIG. 4 Z_(M) /Z_(T) =4 and β=1, see below for definition of β. As can be learned from FIG. 4 the peak of W/W_(k) is within the interval 0.35-0.75, preferably 0.4-0.6, of T_(M) /T_(T). In said preferred interval the degree of efficiency W/W_(k) is well over 90%. The highest degree of efficiency is achieved when T_(M) /T_(T) =0.5. Thus the optimum design according to the present invention is when T_(M1) is equal to T_(M2) and T_(T1) is equal to T_(T2).

When using the findings according to this invention as regards the impedance ratio Z_(M) /Z_(T) and the time ratio T_(M) /T_(T) in dimensioning work it is also necessary to introduce a parameter named β. Said parameter β=2L_(H) k₁ /A_(T2) E_(T2), where L_(H) =L_(T2) +L_(M2) ; k₁ is the constant illustrated in FIG. 2; A_(T2) is the cross-sectional area of the portion 11b; and E_(T2) is the Youngs' modulus for the portion 11b.

In FIG. 5 the relationship of the degree of efficiency W/W_(k) versus the parameter β is shown. When setting up the diagram of FIG. 5 Z_(M) /Z_(T) =4 and T_(M) /T_(T) =0.5. From FIG. 5 it can be learned that the degree of efficiency W/W_(k) decreases for an increasing value of β. Therefore it is important that proper matching values for L_(H) and A_(T2) are chosen and also that a material having a proper Youngs' modulus E_(T2) is chosen. For practical reasons it is not possible to give β too small of a value although the degree of efficiency W/W_(k) increases for a decreasing value of β.

A very important favorable feature of the present invention is that the piston and the drill bit of a hammer device according to the present invention are not subjected to any tensile stresses worth mentioning during the rock crushing work period of the stress wave. Thus the original stress wave can be reflected several times within the system without generating any tensile stress waves worth mentioning. In FIG. 6 the highest positive (tensile) stress (i.e., the portion of the graph disposed above the zero value) and the highest negative (compressive) stress, i.e., the portion of the graph disposed below zero in every cross-section of the piston 10 and drill bit 11 are shown. In the diagram the shown stresses are dimensionless since they are related to a reference stress. From FIG. 6 it can be seen that generally only the piston 10 is subjected to any tensile stresses and that the value of said stresses is negligeable. It should be pointed out that since tensile stresses are almost absent in the piston and drill bit according to the present invention said details will have a longer life than corresponding details in a conventional down-the-hole hammer. It is the tensile stresses that give rise to fatigue of details of that kind.

The diagrams according to FIGS. 3, 4 ,5 and 6 have been set up by using a computer program simulating percussive rock drilling. However, the computer program has only been used to verify the theories of the present invention, namely to have a reversed design of the piston 10 and the drill bit 11.

It should be pointed out that the present invention is in no way restricted to a down-the-hole hammer but is also applicable in e.g. so called impact breakers and hard rock excavating machines. Generally speaking the invention can be used in a piston-drill bit system where the piston is acting directly upon the drill bit. Also there is no limitation concerning the activation of the piston. This means that such activation can be effected by e.g. a hydraulic medium, by air or by any other suitable means.

Also the invention is in no way restricted to the embodiment described above but can be varied freely within the scope of the appending claims. 

I claim:
 1. In a hammer device comprising a drill bit disposed at a front end of the device, and a piston mounted longitudinally behind said drill bit for reciprocation in a longitudinal direction to repeatedly strike said drill bit, the improvement wherein:said drill bit includes front and rear portions of difference impedance, and said piston includes front and rear portions of different impedance, wherein: Z_(M1) /Z_(T1) is in the range of 3.0-5.5, and Z_(M2) /Z_(T2) is in the range 3.0-5.5, where: Z_(M1) is the impedance of said piston rear portion, Z_(T1) is the impedance of said piston front portion, Z_(M2) is the impedance of said drill bit front portion, and Z_(T2) is the impedance of said drill bit rear portion.
 2. A hammer device according to claim 1, wherein:Z_(M1) /Z_(T1) is in the range 3.5-4.5, and Z_(M2) /Z_(T2) is in the range 3.5-4.5.
 3. A hammer device according to claim 2, wherein:

    Z.sub.M1 /Z.sub.T2 =Z.sub.M2 /Z.sub.T2 =4.0.


4. A hammer device according to claim 1, wherein Z_(M1) =Z_(M2), and Z_(T1) =Z_(T2).
 5. A hammer device according to claim 4, wherein: T_(M1) =T_(M2), and T_(T1) =T_(T2).
 6. A hammer device according to claim 1, wherein each of said drill bit front and rear portions and each of said piston front and rear portions has a time parameter T defined as:

    T=L/c

where: L is the longitudinal length of the respective portion, and c is the elastic wave speed of the respective portion, wherein: T_(M1) /T_(T1) is in the range 0.35-0.75, and T_(M2) /T_(T2) is in the range 0.35-0.75, and where: T_(M1) is the time parameter of the piston rear portion, T_(T1) is the time parameter of the piston front portion, T_(M2) is the time parameter of the drill bit front portion, and T_(T2) is the time parameter of the drill bit rear portion.
 7. A hammer device according to claim 6, wherein:T_(M1) /T_(T1) is in the range 0.4-0.6, and T_(M2) /T_(T2) is in the range 0.4-0.6.
 8. A hammer device according to claim 7, wherein:

    T.sub.M1 /T.sub.T1 =T.sub.M2 /T.sub.T2 =0.5.


9. A hammer device according to claim 1, wherein said hammer device is a down-the-hole hammer device.
 10. A piston for use in a hammer device for being reciprocated longitudinally into striking engagement with a drill bit located in front of the piston, said piston including front and rear portions of different impedance, wherein:Z_(M1) /Z_(T1) is in the range 3.0-5.5, where: Z_(M1) is the impedance of said piston rear portion, and Z_(T1) is the impedance of said piston front portion.
 11. A piston according to claim 10, wherein:Z_(M1) /Z_(T1) is in the range 3.5-4.5.
 12. A piston according to claim 11, wherein:

    Z.sub.M1 /Z.sub.T1 =4.0.


13. A piston according to claim 10, wherein each of said piston front and rear portions has a time parameter T defined as:

    T=L/c

where: L is the longitudinal length of the respective portion, and c is the elastic wave speed of the respective portion, wherein: T_(M1) /T_(T1) is in the range 0.35-0.75, where: T_(M1) is the time parameter of the piston rear portion, and T_(T1) is the time parameter of the piston front portion.
 14. A piston according to claim 13, wherein:T_(M1) /T_(T1) is in the range 0.4-0.6.
 15. A piston according to claim 14, wherein:

    T.sub.M1 /T.sub.T1 =0.5.


16. A drill bit for use in a hammer device for being repeatedly struck by a longitudinally reciprocating piston located behind said drill bit, said drill bit including front and rear portions of difference impedance, wherein:Z_(M2) /Z_(T2) is in the range 3.0-5.5, where: Z_(M2) is the impedance of said drill bit front portion, and Z_(T2) is the impedance of said drill bit rear portion.
 17. A drill bit according to claim 16, wherein:Z_(M2) /Z_(T2) is in the range 3.5-4.5.
 18. A drill bit according to claim 17, wherein:

    Z.sub.M2 /Z.sub.T2 =4.0.


19. A drill bit according to claim 16, wherein each of said drill bit front and rear portions has a time parameter T defined as:

    T=L/c

where: L is the longitudinal length of the respective portion, and c is the elastic wave speed of the respective portion, wherein: T_(M2) /T_(T2) is in the range 0.35-0.75 where: T_(M2) is the time parameter of the drill bit rear portion, and T_(T2) is the time parameter of the drill bit front portion.
 20. A drill bit according to claim 19, wherein:T_(M2) /T_(T2) is in the range 0.4-0.6.
 21. A drill bit according to claim 20, wherein:

    T.sub.M2 /T.sub.T2 =0.5. 